A proposed UK law that would require people to opt-in to view adult content online is to be debated in the House of Lords.

Under the proposals anyone who wants to access adult content online would have to specifically tell their ISP or mobile phone operator they want to view it when signing up for an internet connection.

The Online Safety Bill also proposes that ISPs and mobile operators be prohibited from providing access to such content if they don't have to have a mechanism in place to verify the person signing up for the internet connection was aged over 18.

Unless these criteria were met ISPs or mobile operators they would have to install filtering technology that provided an internet connection that blocked access to all adult material.

The bill also proposes that manufacturers of electronic devices provide customers with a means of filtering this content from online material.

The proposal is a private members bill, put forward by Baroness Howe. Such bills do not pass into law without the support of government.

UK foreign secretary William Hague has said that the government does not support default filtering of adult material online, in response to a recent similar proposal from MP Claire Perry.

The government has instead agreed a code of conduct with major UK ISPs that requires them to flag up parental control options to customers.

Dr Mark Watts, partner at law firm Bristows and IT specialist, said that the main difficulty in enacting such a law would be defining exactly what adult material is.

"Trying to write a law that successfully draws a clear line in an area that can be quite subjective is very difficult," he said, adding that it would be better to leave it to ISPs to devise their own approach to filtering online content.